This invention relates to a chemical reinforced glass substrate and an information recording medium including the glass substrate and also to a method of manufacturing the chemical reinforced glass substrate and the information medium.
As one of information recording media, a magnetic disk mounted on a hard disk drive (HDD) is known in the art.
Recently, it is strongly required to increase storage capacity of the magnetic disk. As a result, an extension of a recording region on the magnetic disk and a high recording density of the recording become an emergency matter.
The hard disk drive includes a magnetic head that faces a recording surface of the magnetic disk and flies over the recording surface to write/read information to/from the magnetic disk. It is desirable that a flying height or an interval between the magnetic head and the recording surface is lower and lower because the recording density of the magnetic disk can be increased.
Two methods are known as a driving method for driving the hard disk drive. One is a CSS (Contact Start and Stop) method and the other is an LUL (Load/Unload) method. Because the LUL method permits a reduction of the interval between the magnetic head and the recording surface in comparison with the CSS method, the former enables an increase of the recording density in comparison with the CSS method.
On the other hand, the recording surface of the magnetic disk must be flat and smooth to stabilize a flight of the magnetic head. That is, the magnetic disk must have a substrate that has a flat and smooth surface. As the substrate, attention has been directed to a glass substrate for the magnetic disk because its main surface can be made very flat and very smooth.
The main surface of the glass substrate is polished by a soft polisher to even or smooth them. However, polishing by the use of the soft polisher causes a surface down or a surface rise to occur at an outer edge portion and/or an inner edge portion of the glass substrate. In the LUL method, the magnetic head stays at the outside in a radial direction of the magnetic disk when the magnetic disk is not driven. When the magnetic disk is driven, the magnetic head moves toward the center of the magnetic disk to write/read information to/from the magnetic disk and faces the recording surface of the magnetic disk. Accordingly, the surface down and/or the surface rise at the outer edge portion make the flight of the magnetic head unstable. In the worst case, the magnetic head clashes with the surface of the magnetic disk. In addition, the down and/or the rise limits the recording area of the magnetic disk. This is because an extent of the recording area depends on a flat area of the main surface of the glass substrate.
To solve the above-mentioned problems, several proposals have already been made. For example, a technique for reducing the surface down and the surface rise is disclosed in Japanese Unexamined Patent Publication (JP-A) No. H05-89459. The technique provides appropriate polishing conditions (i.e. polishing pressure and polishing time). Moreover, another technique for stabilizing the flight of the magnetic head is disclosed in Japanese Unexamined Patent Publication (JP-A) No. H05-290365. The technique provides an appropriate radius of curvature at the outer edge portion of the glass substrate.
By the way, the glass substrate is often subjected to chemical reinforcement or treatment used by chemical solution after the polishing process to improve mechanical strength and durability. Such a glass substrate will be called a chemical reinforced glass substrate hereinafter. The chemical reinforcement partially replaces specific ions included in the surface of the glass substrate with other ions larger than the specific ions in ionic radii.
According to the inventors"" experimental studies, it has been found out that the chemical reinforced glass substrate can not accomplish a stable flight of a magnetic head due to head crashes and the like and therefore makes it impossible to widen a recording area of the magnetic recording medium manufactured from the chemical reinforced glass substrate.
It is therefore an object of this invention to provide a chemical reinforced glass substrate which is subjected to chemical reinforcement and which has desirable edge shapes or profiles at outer and inner edges.
It is another object of this invention to provide a chemical reinforced glass substrate of the type described having a high mechanical strength and a flat and smooth surface.
It is still another object of this invention to provide a chemical reinforced glass substrate which can widen a recording area formed on its main surface.
It is further still another object of this invention to provide a chemical reinforced glass substrate used for an information recording medium which can be properly clamped by a clamp for an information recording apparatus.
It is yet another object of this invention to provide a chemical reinforced glass substrate used for an information recording medium which enables a writing/reading head to fly stably over its recording surface.
The ski-jump portions at the outer edge portion of the glass substrate make the flight of the magnetic head unstable and limit the recording area. In addition, it is unavoidable to enlarge the interval between the magnetic head and the recording surface of the magnetic disk because the ski-jump portions at the outer edge portion might bring about the head clashes with the magnetic disk.
On the other hand, the ski-jump portions at the inner edge portion of the glass substrate inclines the magnetic disk against a clamp for clamping the magnetic disk. There is a case where the ski-jump portions at the inner edge portion distort or destroy the magnetic disk when it is clamped by the clamp.
Herein, description will be made about principles of this invention for a better understanding of this invention. Heretofore, a chemical reinforced glass substrate is rarely used for manufacturing a magnetic recording medium or has not been investigated about characteristics of the chemical reinforced glass substrate. According to the inventors"" experimental studies, it has been found out that the magnetic recording medium which includes a chemical reinforced glass substrate can not accomplish a stable flight of a magnetic head and often causes head crashes to occur during read/write operation. As a result, it is difficult to accomplish a low flight operation of the magnetic head and to expand a recording area of the magnetic recording medium.
Further inventors"" research has revealed that the glass substrate is undesirably deformed at inner and outer edge portions of a main surface when the glass substrate is subjected to chemical reinforcement. It has been also found out that deformed portions appear in the form of projections or recesses at the inner and the outer edge portions. In any event, such projections and recesses provide inner and/or outer edge profiles and will be referred to as ski-jump portions and roll-off portions, respectively. The ski-jump portions at the outer edge portion of the glass substrate make the flight of the magnetic head unstable and limit the recording area. In addition, it is unavoidable to enlarge the interval between the magnetic head and the recording surface of the magnetic disk because the ski-jump portions at the outer edge portion might bring about the head clashes with the magnetic disk.
On the other hand, the ski-jump portions at the inner edge portion of the glass substrate inclines the magnetic disk against a clamp for clamping the magnetic disk. This means that the ski-jump portions at the inner edge portion distort or destroy the magnetic disk when it is clamped by a clamp.
The ski-jump portions at the outer edge portion of the glass substrate make the flight of the magnetic head unstable and limit the recording area. In addition, it is unavoidable to enlarge the interval between the magnetic head and the recording surface of the magnetic disk because the ski-jump portions at the outer edge portion might bring about the head clashes with the magnetic disk.
According to a first aspect of this invention, a method is for use in manufacturing a glass substrate for an information recording medium, the glass substrate having an edge portion adjacent to an outer and/or an inner peripheral side end. The method comprises the steps of previously finding a relationship between chemical reinforcement conditions and that profile variation at the edge portion of the glass substrate which results from chemical reinforcement and performing the chemical reinforcement of the glass substrate on the basis of the relationship to obtain a chemically reinforced glass substrate.
As mentioned before, consideration is previously made about the relationship between the profile variation caused by the chemical reinforcement and the chemical reinforcement conditions. The profile variation may occur, for example, on the edge portion adjacent to the outer peripheral side end and may appear in a thickness direction. In this event, the profile variation may be represented by a variable component in the thickness direction. At any rate, the chemical reinforcement is performed on the basis of the relationship previously detected or found and, as a result, the profile variation on the edge portion adjacent to the outer peripheral side end can be controlled by the chemical reinforcement condition. This applies to the edge portion adjacent to the inner peripheral side end.
The chemical reinforcement may be performed, for example, by a first method of chemically reinforcing a glass substrate by using ion exchange and by a second method of chemically reinforcing the glass substrate by using a de-alkali process. Specifically, the first method realizes the chemical reinforcement of the glass substrate by exchanging ions included in a surface layer of the glass substrate for ions which are included in a chemical reinforcement solution and which have diameters greater than the ions of the surface layer. With this method, expansion takes place in an in-plane direction of the glass substrate and a profile variation component on the main surface is represented by a positive value. Such a positive value of the profile variation component brings about a surface rise area.
On the other hand, the second method which uses the de-alkali process shrinks the glass substrate in the in-plane direction and the profile variation component is represented by a negative value. This brings about a surface down area. In addition, it is confirmed that the profile variation component of the edge portion adjacent to the inner peripheral side end is smaller than that of the outer peripheral side end by 10%-20%.
According to a second aspect of this invention, a method is for use in manufacturing a glass substrate for an information recording medium. The glass substrate has an edge portion adjacent to an outer and/or an inner peripheral side end. The method comprises the steps of previously finding a relationship between chemical reinforcement conditions and that profile variation at the edge portion of the glass substrate which results from chemical reinforcement, deciding a profile on the edge portion by predicting the profile variation of the edge portion to obtain, as a glass substrate prior to chemical reinforcement, a glass substrate prior to chemical reinforcement which has a decided profile and which is not subjected to the chemical reinforcement, and performing the chemical reinforcement of the glass substrate prior to chemical reinforcement to obtain the glass substrate which has a desired profile at the edge portion.
In addition to the merits mentioned in conjunction with the first aspect, the second aspect of this invention predicts the profile variation components caused by the chemical reinforcement and uses the glass substrate prior to chemical reinforcement that can cancel them. With this method, it is possible to strictly and precisely control an outer peripheral contour of the glass substrate subjected to the chemical reinforcement. This applies to an inner peripheral contour of the glass substrate.
According to a third aspect of this invention, the chemical reinforcement performing step is performed on conditions such that the profile variation on the edge portion becomes small. For example, the chemical reinforcement is carried out so that the profile variation becomes small on the outer peripheral side end. In this case, it is possible to suppress, on the outer peripheral side end, the profile variation which might occur due to the chemical reinforcement, if the glass substrate prior to chemical reinforcement is flat on the outer peripheral side end.
Alternatively, even when use is made of the glass substrate prior to chemical reinforcement which can cancel the profile variation, as mentioned In the second aspect, a profile variation extremely becomes small between the glass substrate prior to chemical reinforcement and the glass substrate. This means that the outer edge profile can readily be controlled as compared with a large profile variation and stable processing can be executed with the profile variation kept small. These apply to the inner peripheral side end of the glass substrate.
According to a fourth aspect of this invention, the chemical reinforcement performing step is performed under the chemical reinforcement condition such that a compressive stress layer formed on a surface layer of the glass substrate by the chemical reinforcement reaches to a depth between 3 and 100 xcexcm and has a compressive stress of 1-15 kg/mm2 and that a tensile stress caused by the chemical reinforcement within the glass substrate is not larger than 4.5 kg/mm2.
As mentioned before, the compressive stress layer at first reaches to the depth between 3 and 100 xcexcm. This makes it possible to keep desirable mechanical strength of the glass substrate and to reduce the profile variation component on the outer peripheral side end when the chemical reinforcement is performed.
When the depth of the compressive stress layer is thinner than 3 xcexcm, the mechanical strength becomes undesirably weak in durability and against breakage. When the depth of the compressive stress layer exceeds 100 xcexcm, the profile variation component becomes large when the chemical reinforcement is performed. Preferably, the depth of the falls within a range between 40 and 80 xcexcm and more preferably, within a range between 50 and 70 xcexcm.
Second, the compressive stress caused in the surface layer of the glass substrate by the chemical reinforcement is selected between 1 and 15 kg/mm2 while the tensile stress caused within the glass substrate is not greater than 4.5 kg/mm2. This serves to improve the strength of the glass substrate and the durability against breakage based on aging. The compressive stress less than 1 kg/mm2 undesirably weakens the strength of the glass substrate (deterioration of the durability against defects and the characteristics withstanding breakage) while the compressive stress over 15 kg/mm2 enlarges the profile variation components and makes it difficult to control the outer edge profile.
The tensile stress over 4.5 kg/mm2 also enlarges the profile variation components and make the control of the outer edge profile difficult
At any rate, the above-mentioned merits are more excellent by setting the depth of the compressive stress layer, the compressive stress, and the tensile stress into optimum values. These are true of the inner peripheral side end.
According to a fifth aspect of this invention, the chemical reinforcement condition defines a processing temperature and a processing time during the chemical reinforcement. By rendering the processing temperature and the processing time of the chemical reinforcement condition into predetermined ranges, it is possible to reduce the profile variation components which appear on the outer and/or the inner peripheral side ends during the chemical reinforcement processing.
In addition to the processing temperature and time, the chemical reinforcement condition may be specified by a species of fused salts and a mixing ratio of the fused salts. However, the processing temperature and time can be readily adjusted in comparison with the species and the mixing ratio of the fused salts. Accordingly, controlling the processing temperature and time is very effective on massproduction and in workability.
According to a sixth aspect of this invention, it is preferable that the processing temperature and the processing time fall with a range between 280xc2x0 C. and 400xc2x0 C. and a duration between 0.5 and 5 hours, respectively. If the processing temperature is lower than 280xc2x0 C., the processing temperature is undesirably lower than a melting point of the fused salt or salts. On the other hand, the processing temperature higher than 400xc2x0 C. undesirably shortens the processing time and gives rise to a reduction of workability. The processing time shorter than 0.5 hour becomes worse in workability while the processing time over 5 hours undesirably worsen productivity.
Preferably, the processing temperature and the processing time may fall within ranges between 340 and 360xc2x0 C. and between 1 and 4 hours, respectively, so as to lower the profile variation components on the outer and/or the inner peripheral side ends of the chemically reinforced glass substrate, although they can not be uniquely determined because of depending upon glass compositions of the glass substrate, compositions of the chemical reinforcement solution, and so on.
According to a seventh aspect of this invention, the glass substrate prior to chemical reinforcement has a main surface chamfered and polished together with the edge portion adjacent to the outer and/or the inner peripheral side end. In addition, the previously finding step previously finds or predicts, as the relationship, a relationship between a polishing condition of the main surface and an edge profile obtained on the basis of the polishing condition. Moreover, the deciding step obtains the glass substrate prior to chemical reinforcement by controlling the polishing condition of the main surface on the basis of the above-mentioned relationship between the polishing condition and the edge profile. Specifically, the glass substrate prior to chemical reinforcement has the main surface chamfered along the outer and/or the inner peripheral side ends each adjacent to the edge portion. Prediction is made about the relationship between the polishing condition of polishing the main surface of the glass substrate subjected to a chamfering process and the profile of the edge portion adjacent to the outer and/or the inner peripheral side end. It is posssible to obtain the glass substrate prior to the chemical reinforcement, (may be called an unreinforced or a provisional glass substrate), which has desired outer and/or inner edge profiles by controlling the polishing condition of the main surface on the basis of the above-mentioned relationship.
According to an eighth aspect of this invention, the polishing condition is determined such that the edge portion is polished to be put into a surface down state lowered relative to the main surface 2 of the glass substrate 1, as Illustrated in FIG. 1. Such a polishing condition of rendering the main surface 2 into the surface down state makes it possible to simply and precisely obtain the provisional glass substrate which has the outer edge profile removed by or cancelled by a profile variation caused by the chemical reinforcement. This is true of the inner peripheral side end of the provisional glass substrate.
According to a ninth aspect of this invention, the polishing condition determined for the surface down state is defined such that use is made about a soft polisher of a hardness between 60 and 80 (Askers-C) and a surface pressure to the glass substrate is kept at a range between 40 and 150 kg/cm2 during polishing. The above-mentioned polishing condition makes it possible to readily and precisely attain the glass substrate prior to the chemical reinforcement, which stably keeps the surface down state and to readily control the outer edge profile. In addition, it has been found out that the outer edge profile of the edge portion tends to be rendered into the surface down state as the polisher or a polishing pad is hardened with the other conditions kept intact. The outer edge profile is liable to become the surface rise state as the polishing pressure becomes high while the outer edge profile is rendered into the surface down state as the polishing rotation speed becomes high.
The outer edge profile is varied in dependency upon polishing conditions determined by a structure, a size, and an amount of abrasive materials of a polishing machine. However, controlling the outer edge profile by the hardness of the polisher has a good controllability and is readily executed over a wide range. Under the circumstances, it is preferable that the outer edge profile may be mainly controlled by the hardness of the polisher and subordinately controlled by the polishing pressure and the polishing speed.
According to a tenth aspect of this invention, a glass substrate is subjected to chemical reinforcement and is for use in an information recording medium. The glass substrate chemically reinforced has a main surface and an edge portion which is adjacent to an outer and/or an inner peripheral side end and which is contiguous to the main surface. The edge portion of the glass substrate Which is chemically reinforced has a predetermined region defined by a profile which falls within xc2x10.35 xcexcm in relation to a flat portion of the main surface determined as a reference surface (zero). With this structure, it is possible to make a magnetic head stably float and run without any head crashes with a low height left and to widen a recording area. At any rate, high density recording and reproducing can be achieved. This means that the glass substrate after the chemical reinforcement is kept flat in the outer edge profile to the exent that no problem takes place in connection with the extension of the recording area and the high density recording and reproducing. This is very effective in a magnetic recording medium of a LUL type.
In the meanwhile, the predetermined region adjacent to the outer peripheral side end may be optionally determined along the outer peripheral side end. However, it is preferable that the predetermined region may be defined by a region which is largely deviated from a reference surface which is determined by a flat portion of the main surface and which is roughened in flatness.
Specifically, it is possible to define, as the predetermined region, a region between an outer periphery of a recording area (an area of the main surface usually keeping flatness) on the main surface and an outermost periphery of a glide region determined on the main surface. Alternatively, the predetermined region may be determined by a region from a side end wall of the glass substrate to an inside of the outer periphery of the recording area.
Herein, it is to be noted that the recording area is generally included within the glide region but may be identical with the glide region.
At any rate, It is preferable that the outer edge profile falls within a range of xc2x10.20 xcexcm (namely, between xe2x88x920.20 xcexcm and +0.20 xcexcm) and more preferably, a range of xc2x10.10 xcexcm (xe2x88x920.10 xcexcm and +0.10 xcexcm). This also applies to the inner peripheral side end.
According to an eleventh aspect of this invention, the glass substrate is subjected to chemical reinforcement and defined as a glass substrate after chemical reinforcement. The main surface of the glass substrate after chemical reinforcement has a glide area which includes a recording area located inside of the glide area. The glide area has a glide outer periphery while the recording area has a recording area outer periphery inside the glide outer periphery and a flat area. The edge profile chemically reinforced has, within an area extended from the glide outer periphery to an inside of the recording area, a ski-jumped point which is the highest point with respect to a reference surface (zero) defined by the flat area and which has a ski-jump value not greater than xc2x10.35 xcexcm at the ski-jump point. In addition, the edge profile also has, at a roll-off point defined by a position of the glide outer periphery, a roll-off value not greater than xc2x10.35 xcexcm with respect to the reference surface.
As mentioned above, the ski-jump point and the roll-off point are defined in the area which is extended from the glide outer periphery to an inside of the recording area. This means that it is possible to render the ski-jump value and the roll-off value into less than +0.35 xcexcm and more than xe2x88x920.35 xcexcm, respectively, with respect to the reference surface. The edge profile which has the above-mentioned ski-jump value and roll-off value can attain the merits mentioned with reference to the tenth aspect before. In addition, it Is possible to readily manage products by executing numerical control in consideration of the ski-jump point and the roll-off point.
Specifically, the ski-jump value represents a value of the ski-jump point which is the highest point on the edge profile, with respect to the reference to the flat surface of the glass substrate while the roll-off value represents a value of the roll-off point determined on a border line drawn at a position of the glide outer periphery, as mentioned before. The roll-off value is also decided with respect to the reference surface.
Both the ski-jump value and the roll-off value are measured in the following manner.
As shown in FIG. 2, consideration is made about a section of the glass substrate cut by a plane which is perpendicular to the main surface of the glass substrate and which passes through a center of the glass substrate of a disk shape. Within the section, two reference points are determined within the recording area of the main surface on an outline of the recording area and are successively named R1 and R2 from the order near to the center of the glass substrate. In addition, an additional point R3 is determined on a line extended from the recording area outer periphery in an outer direction and is remote from the recording area outer periphery by a predetermined distance. The additional point R3 defines a position of the glide outer periphery of the glide area.
Next, the reference points R1 and R2 are connected to each other by a line which is extended outwards of the glass substrate. The extended line is drawn by a broken line in FIG. 2. Within an area between R2 and R3, measurement is made about a distance between the line R1R2 (or the extended line) and each point set on the outline of the glass substrate. The ski-jump point (represented by S) on the outline of the glass substrate is defined by a highest point at which the distance is the highest in a positive direction. The distance s at the ski-jump point is the ski-jump value.
On the other hand, the roll-off point is defined by a point R which corresponds to R3 and which is placed on the outline of the glass substrate while a distance r between point R and the straight line R1R2 (or the extened line) is representative of the roll-off value.
As shown in FIG. 3, it happens that the ski-jump value s slightly takes a negative value. In this case, the ski-jump specifies the surface down state. As shown in FIG. 4, the roll-off value r also often takes a positive value, which specifies a surface rise state of the glass substrate. Moreover, the ski-jump value s may become equal to the roll-off value r, as shown in FIG. 4.
The reference points R1 and R2 and the point R3 may be optionally selected with reference to a size of the glass substrate. For example, when the glass substrate has an outside diameter of 2.5 inches, 3.0 inches, and 3.5 inches, the point R3 may be determined at a position which is placed at 1 mm from the outer peripheral side end on an inside of the glass substrate. In the case of the glass substrate of 2.5 inches (65 mm in diameter), the reference points R1 and R2 and the roll-off point R3 may be determined, for example, at the positions of 23 mm, 27 mm, and 32.5 mm from the center of the glass substrate, respectively. The roll-off point R3 in the above-mentioned example is determined on the outer peripheral side end.
When the ski-jump value exceeds the range of xc2x10.35 xcexcm, the magnetic disk can not accomplish stable floating and gives rise to head crashes. This make it difficult to install the magnetic recording medium within a magnetic disk drive.
The roll-off value over the range of xc2x10.35 xcexcm also deteriorates floating stability of the magnetic head and gives rise to head crashes.
Preferably, each of the ski-jump value and the roll-off value falls within a range of xc2x120 xcexcm and more preferably, within a range of xc2x10.10 xcexcm.
According to a twelfth aspect of this invention, the glass substrate after chemical reinforcement has a compressive stress layer within a surface layer which is caused by the chemical reinforcement and which has a depth between 3 and 100 xcexcm and a compressive stress of 1-15 kg/mm2,
Moreover, the glass substrate after chemical reinforcement also has a tensile stress not greater than 4.5 kg/mm2 within an inside of the glass substrate.
The depth of the compressive stress layer between 3 and 100 xcexcm makes it possible to manufacture an information recording medium glass substrate which has preferable mechanical strength. The depth less than 3 xcexcm weakens the mechanical strength of the glass substrate (durability against defects and characteristic withstanding breakage). When the depth of the compressive stress layer exceeds 100 xcexcm, the profile variation component becomes large when the chemical reinforcement is performed. Preferably, the depth of the falls within a range between 40 and 80 xcexcm and more preferably, within a range between 50 and 70 xcexcm.
Second, the compressive stress caused in the surface layer of the glass substrate by the chemical reinforcement is selected between 1 and 15 kg/mm2 while the tensile stress caused within the glass substrate is not greater than 4.5 kg/mm2. This serves to improve the strength of the glass substrate and the durability against breakage based on aging. The compressive stress less than 1 kg/mm2 undesirably weakens the strength of the glass substrate (deterioration of the durability against defects and the characteristics withstanding breakage) while the compressive stress over 15 kg/mm2 enlarges the profile variation components and makes it difficult to control the outer edge profile.
The tensile stress over 4.5 kg/mm2 also enlarges the profile variation components and make the control of the outer edge profile difficult
At any rate, the above-mentioned merits are more excellent by setting the depth of the compressive stress layer, the compressive stress, and the tensile stress into optimum values. These are true of the inner peripheral side end.
According to a thirteenth aspect of this invention, a method of manufacturing an information recording medium from the glass substrate comprises the step of depositing a recording layer on the main surface of the glass substrate. The information recording medium thus manufactured has the glass substrate flat on the outer edge profile and a wide recording area. In such an information recording medium, the glass substrate has a flat inner edge profile also and can avoid breakage. In any event, the information recording medium can be appropriately mounted onto a magnetic memory device.
According to a fourteenth aspect of this invention, an information recording medium manufactured from the glass substrate comprises a magnetic layer over the main surface. The information recording medium has a high recording density because the glass substrate has a flat outer edge profile and can widen a recording area. The glass substrate which has the flat inner edge profile can avoid breakage. In any event, the information recording medium can be appropriately mounted onto a magnetic memory device.
According to a fifteenth aspect of this invention, the information recording medium is available for a magnetic recording medium of a LUL drive type which can realize an extremely low floating operation of the magnetic head. Therefore, this invention is very effective when it is applied to the LUL drive type magnetic recording medium.